Nitrogen and carbon isotope ratios in termites: an indicator of trophic habit along the gradient from wood-feeding to soil-feeding

1. Nitrogen and carbon stable-isotope ratios (δ¹⁵N and δ¹³C) of body tissues, mound/nest materials and dietary substrates were determined in termite species with differing trophic habits, sampled from the Mbalmayo Forest Reserve, southern Cameroon.
2. δ¹⁵N of termite tissues was enriched gradually along a spectrum of species representing a trophic gradient from wood- to soil-feeding. Species that could be identified from their general biology and from gut content analysis as feeding on well-rotted wood or as wood/soil interface feeders showed δ¹⁵N intermediate between sound-wood-feeders and soil-feeders. It is proposed that δ¹⁵N is therefore a possible indicator of the functional position of species in the humification process. Differences in δ¹³C were also observed between wood-feeding and soil-feeding forms.
3. High values of δ¹⁵N in soil-feeding termites suggest that nitrogen fixation is of little importance in these species.
4. A wide range of isotope effects (the difference in isotope ratios between termites and their diet) was observed for both nitrogen (Δδ¹⁵N = –1.6 to + 8.8‰) and carbon (Δδ¹³C = –2.2 to + 3.0‰), which suggests a diversity of nutrient acquisition mechanisms within termites and diverse relationships between termites and their intestinal micro-organisms.     

Estimation of food composition of Hodotermes mossambicus (Isoptera: Hodotermitidae) based on observations and stable carbon isotope ratios

Abstract The diet of the harvester termite Hodotermes mossambicus was investigated at two sites with distinct dietary components: C₄ grasses (δ¹³C isotope values, ?13.8‰ to ?14.0‰) and C₃ plants (δ¹³C isotope values, ?25.6‰ to ?27.1‰). By comparing observations of food items carried into the colony by the termites and carbon isotope ratios of whole termites (that determined assimilated carbon), the relative proportion of the C₃ and C₄ plant food components of the termite diet was estimated. There was agreement between the observational data and stable carbon isotopic data, with grass representing approximately 93% of the diet of H. mossambicus at two study sites (urban and rural) on the South African highveld. However, when correcting for mass of food items, that is, C₃ and C₄, carried by termites, the proportion of grass (C₄) in the diet may be underestimated.     

Isotope ratios of cellulose from plants having different photosynthetic pathways

Hydrogen and carbon isotope ratios of cellulose nitrate and oxygen isotope ratios of cellulose from C₃, C₄, and Crassulacean acid metabolism (CAM) plants were determined for plants growing within a small area in Val Verde County, Texas. Plants having CAM had distinctly higher deuterium/hydrogen (D/H) ratios than plants having C₃ and C₄ metabolism. When hydrogen isotope ratios are plotted against carbon isotope ratios, each photosynthetic mode separates into a distinct cluster of points. C₄ plants had many D/H ratios similar to those of C₃ plants, so that hydrogen isotope ratios cannot be used to distinguish between these two photosynthetic modes. Portulaca mundula, which may have a modified photosynthetic mode between C₄ and CAM, had a hydrogen isotope ratio between those of the C₄ and CAM plants. When oxygen isotope ratios are plotted against carbon isotope ratios, no distinct clustering of the C₄ and CAM plants occurs. Thus, oxygen isotope ratios are not useful in distinguishing between these metabolic modes. A plot of hydrogen isotope ratios versus oxygen isotope ratios for this sample set shows considerable overlap between oxygen isotope ratios of the different photosynthetic modes without a concomitant overlap in the hydrogen isotope ratios of CAM and the other two photosynthetic modes. This observation is consistent with the hypothesis that higher D/H ratios in CAM plants relative to C₃ and C₄ plants are due to isotopic fractionations occurring during biochemical reactions.     

Confirmation of soil-feeding termites (Isoptera; Termitidae; Termitinae) in Australia using stable isotope ratios

1. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios of termites (Isoptera) were examined in Darwin, northern Australia. It is suggested that the stable isotope technique, together with phylogenetics, is a useful tool to understand the evolution of functional groups in detritivores.
2. A high δ¹⁵N value was observed in the Termes-Capritermes branch of the subfamily Termitinae and the genus Amitermes , two distinct taxonomic groups that evolved from wood-feeding to soil-feeding in Australia. Among eight Termes-Capritermes branch species, only two species ( Xylochomitermes melvillensis and Ephelotermes melachoma ) were discernible as wood/soil interface feeders, the remaining six species analysed were soil-feeders, where the diet preference was identified by using δ¹⁵N of workers.
3. The Termes-Capritermes group in Australia contains both wood/soil interface feeders and soil-feeders, whereas wood/soil interface feeders in Cameroon are from the Termes-Capritermes branch while soil-feeders are from Cubitermes group. The result confirmed that soil-feeding forms evolved both in Australia and Africa, but from different phylogenetic groups.     

Seasonal variation in pathways of CH4 production and in CH4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors

Flooded rice fields, which are an important source of the atmospheric methane, have become a model system for the study of interactions between various microbial processes. We used a combination of stable carbon isotope measurements and application of specific inhibitors in order to investigate the importance of various methanogenic pathways and of CH₄ oxidation for controlling CH₄ emission. The fraction of CH₄ produced from acetate and H₂/CO₂ was calculated from the isotopic signatures of acetate, carbon dioxide (CO₂) and methane (CH₄) measured in porewater, gas bubbles, in the aerenchyma of the plants and/or in incubation experiments. The calculated ratio between both pathways reflected well the ratio determined by application of methyl fluoride (CH₃F) as specific inhibitor of acetate‐dependent methanogenesis. Only at the end of the season, the theoretical ratio of acetate: H₂ = 2 : 1 was reached, whereas at the beginning H₂/CO₂‐dependent methanogenesis dominated. The isotope discrimination was different between rooted surface soil and unrooted deep soil. Root‐associated CH₄ production was mainly driven by H₂/CO₂. Porewater CH₄ was found to be a poor proxy for produced CH₄. The fraction of CH₄ oxidised was calculated from the isotopic signature of CH₄ produced in vitro compared to CH₄ emitted in situ, corrected for the fractionation during the passage from the aerenchyma to the atmosphere. Isotope mass balances and in situ inhibition experiments with difluoromethane (CH₂F₂) as specific inhibitor of methanotrophic bacteria agreed that CH₄ oxidation was quantitatively important at the beginning of the season, but decreased later. The seasonal pattern was consistent with the change of potential CH₄ oxidation rates measured in vitro. At the end of the season, isotope techniques detected an increase of oxidation activity that was too small to be measured with the flux‐based inhibitor technique. If porewater CH₄ was used as a proxy of produced CH₄, neither magnitude nor seasonal pattern of in situ CH₄ oxidation could be reproduced. An oxidation signal was also found in the isotopic signature of CH₄ from gas bubbles that were released by natural ebullition. In contrast, bubbles stirred up from the bulk soil had preserved the isotopic signature of the originally produced CH₄.     

Methane and hydrogen production in a termite-symbiont system

Methane and hydrogen emission rates and the ¹³C of CH₄ were observed for various termites in Australia, Thailand and Japan. Combined with the already reported emission rates of CH₄ in the literature, the phylogenetic trend was examined. Emission rates of the observed termites were categorized into five groups: group I with high CH₄ and low H₂ emission rates with a CH₄/H₂ ratio of typically 10/1; group II with high CH₄ and high H₂ emissions with a CH₄/H₂ ratio of 4/1–1/2; group III with low emission rates of CH₄ and H₂; group IV with high H₂ and insignificant CH₄ emissions; and group V with insignificant emissions for both CH₄ and H₂. In lower termites, there are both colonies infected and uninfected with methanogens even in the same species, and no specific trend in CH₄ and H₂ emissions was observed within a genus. Whether protozoa in the hindgut of termites are infected with methanogens or not and the differences in species compositions of protozoa are possibly responsible for the inter-colonial variations. The proportions of infected colonies were possibly small for the family Kalotermitidae (dry wood feeders), and relatively large for families of wet or damp wood feeders. The hydrogen emission rate possibly depends on the locality of methanogens: namely, whether they are intracellular symbionts of protozoa or whether they are attached to the hindgut wall. Emission rates of higher termites were classified into groups according to genera and the diet. Most species of soil or wood/soil interface feeders classified into group I, while the soil feeders Dicuspiditermes in Thailand and Amitermes in Australia were classified into groups with high H₂ emission rates. Typical wood-feeding termites and fungus-growing termites were classified into group III. The results indicate that higher termites tend to increase the CH₄ emission rate during dietary evolution from wood- to soil-feeding, and two types of the system with different efficiencies of interspecies transfer of H₂ have been formed. The ¹³C of CH₄ was discernible with a difference in the decomposition process in the termite–symbiont system among lower termites, fungus-growing termites and other higher termites.     

Quantification of methane oxidation in the rhizosphere of emergent aquatic macrophytes: defining upper limits

Rates of rhizospheric methane oxidation were evaluated by aerobic incubations of subcores collected in flooded anoxic soils populated by emergent macrophytes, by greenhouse whole plant incubations, and by CH₄ stable isotopic analysis. Subcore incubations defined upper limits for rhizospheric methane oxidation on an areal basis which were equal to or greater than emission rates. These rates are considered upper limits because O₂ did not limit CH₄ uptake as is likely to occur in situ. The ratio of maximum potential methane oxidation (MO) to methane emission (ME) ranged from 0.7 to 1.9 in Louisiana rice (Oryza sativa), from 1.0 to 4.0 in a N. Florida Sagittaria lancifolia marsh, and from 5.6 to 51 in Everglades Typha domingensis and Cladium jamaicense areas. Methane oxidation/methane emission ratios determined in whole plant incubations of Sagittaria lancifolia under oxic and anoxic conditions ranged from 0.5 to 1.6. Methane oxidation activity associated with emergent aquatic macrophytes was found primarily in fine root material. A weak correlation was observed between live root biomass and CH₄ uptake in Typha. Rhizomes showed small or zero rates of methane uptake and no uptake was associated with plant stems. Methane stable isotope data from a S. lancifolia marsh were as follows: CH₄ emitted from plants: −61.6 ± 0.3%; CH₄ within stems: −42.0 ± 0.2%; CH₄ within sedimentary bubbles: −51.7 ± 0.3%). The ¹³C enrichment observed relative to emitted CH₄ could be due to preferential mobilization of CH₄ containing the lighter isotope and/or the action of methanotrophic bacteria.     

Carbon, hydrogen, and oxygen isotope ratios of cellulose from plants having intermediary photosynthetic modes

Carbon and hydrogen isotope ratios of cellulose nitrate and oxygen isotope ratios of cellulose from species of greenhouse plants having different photosynthetic modes were determined. When hydrogen isotope ratios are plotted against carbon isotope ratios, four clusters of points are discernible, each representing different photosynthetic modes: C₃ plants, C₄ plants, CAM plants, and C₃ plants that can shift to CAM or show the phenomenon referred to as CAM-cycling. The combination of oxygen and carbon isotope ratios does not distinguish among the different photosynthetic modes. Analysis of the carbon and hydrogen isotope ratios of cellulose nitrate should prove useful for screening different photosynthetic modes in field specimens that grew near one another. This method will be particularly useful for detection of plants which show CAM-cycling.     

Stable isotopic analysis of human diet in the Marianas Archipelago,Western Pacific

Proportions of marine vs. terrestrial resources in prehistoric human diets in the southern Mariana Islands (Guam, Rota, Saipan), Micronesia, have been estimated by analysis of stable isotope ratios of carbon and nitrogen in bone collagen and of carbon in apatite. The isotopic composition of marine and terrestrial food resources from the Marianas have also been determined. Experimental evidence shows that collagen carbon isotopes mainly reflect those of dietary protein sources and thus overestimate the contribution of marine animal foods. Marine protein consumption apparently ranges from ∼20% to ∼50% on these islands. Experiments also demonstrate the carbon isotope ratio of bone apatite carbonate accurately reflects that of the whole diet. Carbonate carbon isotope data suggest some individuals consumed significant amounts of ¹³C-enriched (C₄) plants or seaweeds. Sugar cane is an indigenous C₄ crop and seaweeds are eaten throughout the Pacific, but they have not been considered by archaeologists to have been prehistoric dietary staples. Apatite carbon isotope analysis has apparently identified previously unrecognized prehistoric dietary adaptations in the Mariana Islands, but this must be confirmed by archaeobotanical evidence. Am J Phys Anthropol 104:343–361, 1997. © 1997 Wiley-Liss, Inc.     

Respiratory gas exchanges of termites from the Sabah (Borneo) assemblage

Abstract.Oxygen uptake and carbon dioxide release at 28°C were determined in worker castes of twenty-six species of forest termites from the Danum Valley Conservation Area, south-east Sabah, by Warburg manometry. Metabolic rate varied inversely with body weight in a suite of soil-, wood/soil- and wood-feeding species, giving a slope (in a log–log plot) of – 0.63. However, a number of large species, actively foraging forms such as Macrotermes malaccensis, M. gilvus, Havilanditermes atripennis and Hospitalitermes hospitalis, but also the wood-feeding Schedorhinotermes sarawakensis, showed an oxygen consumption greater than expected for their body weight. Rates of methane emission were above 0.100 μmol g^–1 h^–1 in seventeen species, with very high fluxes in two wood/soil-feeders, Termes borneensis (0.546 ± 0.163 μmol g^–1 h^–1) and Prohamitermes mirabilis (0.303 ± 0.123 μmol g^–1 h^–1). Of the fifteen remaining species, seven were soil-feeders, five were wood-feeders, two were wood/litter-feeders and a single species fed on lichen and moss. Low or negligible CH₄ emissions (< 0.100 μmol g^–1 h^–1) were observed in three other species, all wood-feeders. An apparent respiratory quotient (RQ_app) was calculated using xCO₂ and xO₂ (corrected for methane emission, but not hydrogen). Mean RQ_app was at or above 1.00 in eleven species and between 0.95 and 1.00 in a further six species, the two sets of species together representing all trophic groups, including lichen-feeders. This is argued to be consistent with carbohydrate being the principal substrate supporting respiration.     

Radiocarbon dating of methane and carbon dioxide evaded from a temperate peatland stream

Streams draining peatlands export large quantities of carbon in different chemical forms and are an important part of the carbon cycle. Radiocarbon (¹⁴C) analysis/dating provides unique information on the source and rate that carbon is cycled through ecosystems, as has recently been demonstrated at the air–water interface through analysis of carbon dioxide (CO₂) lost from peatland streams by evasion (degassing). Peatland streams also have the potential to release large amounts of methane (CH₄) and, though ¹⁴C analysis of CH₄ emitted by ebullition (bubbling) has been previously reported, diffusive emissions have not. We describe methods that enable the ¹⁴C analysis of CH₄ evaded from peatland streams. Using these methods, we investigated the ¹⁴C age and stable carbon isotope composition of both CH₄ and CO₂ evaded from a small peatland stream draining a temperate raised mire. Methane was aged between 1617 and 1987 years BP, and was much older than CO₂ which had an age range of 303–521 years BP. Isotope mass balance modelling of the results indicated that the CO₂ and CH₄ evaded from the stream were derived from different source areas, with most evaded CO₂ originating from younger layers located nearer the peat surface compared to CH₄. The study demonstrates the insight that can be gained into peatland carbon cycling from a methodological development which enables dual isotope (¹⁴C and ¹³C) analysis of both CH₄ and CO₂ collected at the same time and in the same way.     

Carbon isotope composition of CH4 from rice paddies in Japan

Carbon isotope ratios (¹³C) for bubble CH₄ in a submerged paddy soil were studied in Yokohama, Japan, throughout a growing period, and its variation was found. Bubble CH₄ collected from other 33 paddy fields in Japan was also measured for its ¹³C and the results agreed with Yokohama. Furthermore, the variation occurred irrespective of the amount and the type of supplied organic substances to the fields (whole rice straw, rice stubble, or compost). The ¹³C value (average value of -55.9 ± 4.24) from these paddy fields was higher than those of the CH₄ emitted from African and North American paddies. The higher value was little affected by their difference in the supplied organic substances. CH₄ oxidation likely occurs for bubble CH₄ in the shallow paddy fields. A rough estimate of the total CH₄ production, using isotope mass balance, showed that 17 to 22% of organic carbon supplied to Japanese paddies transforms to CH₄.     

Nitrogen Mineralization, Ammonia Accumulation, and Emission of Gaseous NH3 by Soil-feeding Termites

There are numerous reports on the accumulation of ammonia in the mounds of soil-feeding termites. Here, we provided direct evidence for an effective mineralization of nitrogenous soil organic matter in the gut of Cubitermes spp., which gives rise to enormous ammonia concentrations in the intestinal tract. In Cubitermes ugandensis, the ammonia content of the nest material [24.5 μmol (g dry wt.)⁻¹] was about 300-fold higher than that of the parent soil. Large amounts of ammonia were present throughout the intestinal tract, with lowest values in the extremely alkaline gut sections (pH >12) and highest values posterior hindgut [185 μmol (g dry wt.)⁻¹]. Results obtained with other Cubitermes species were similar. Ammonia concentrations in the posterior hindgut of these humivorous species (up to 130 mM) are among the highest values ever reported for soil macroinvertebrates and are matched only by insects feeding on an extremely protein-rich diet (e.g., the sarcophageous larvae of blowflies). Volatilization of ammonia [about 10 nmol (g fresh wt.)⁻¹ h⁻¹], either directly by emission from the termite body or indirectly from their feces, led to NH₃ concentrations in the nest atmosphere of C. ugandensis that were three orders of magnitude above the ambient background – a relative accumulation that is considerably higher than that observed with CH₄ and CO₂. Together with previous results, these observations document that through their feeding activity and due to the physicochemical and biochemical properties of their digestive system, soil-feeding termites effectively catalyze the transformation of refractory soil organic nitrogen to a plant-available form that is protected from leaching by adsorption to the nest soil. Nitrogen mineralization rates of soil-feeding termites may surpass those effected by tropical earthworms and should contribute significantly to nitrogen fluxes in tropical ecosystems.     

Effects of vegetation on the emission of methane from submerged paddy soil

Summary Methane emission rates from rice-vegetated paddy fields followed a seasonal pattern different to that of weed-covered or unvegetated fields. Presence of rice plants stimulated the emission of CH₄ both in the laboratory and in the field. In unvegetated paddy fields CH₄ was emitted almost exclusively by ebullition. By contrast, in rice-vegetated fields more than 90% of the CH₄ emission was due to plant-mediated transport. Rice plants stimulated methanogenesis in the submerged soil, but also enhanced the CH₄ oxidation rates within the rhizosphere so that only 23% of the produced CH₄ was emitted. Gas bubbles in vegetated paddy soils contained lower CH₄ mixing ratios than in unvegetated fiels. Weed plants were also efficient in mediating gas exchnage between submerged soil and atmosphere, but did not stimulate methanogenesis. Weed plants caused a relatively high redox potential in the submerged soil so that 95% of the produced CH₄ was oxidized and did not reach the atmosphere. The emission of CH₄ was stimulated, however, when the cultures were incubated under gas atmospheres containing acetylene or consisting of O₂-free nitrogen.     

Diversity of active aerobic methanotrophs along depth profiles of arctic and subarctic lake water column and sediments

Methane (CH₄) emitted from high-latitude lakes accounts for 2–6% of the global atmospheric CH₄ budget. Methanotrophs in lake sediments and water columns mitigate the amount of CH₄ that enters the atmosphere, yet their identity and activity in arctic and subarctic lakes are poorly understood. We used stable isotope probing (SIP), quantitative PCR (Q-PCR), pyrosequencing and enrichment cultures to determine the identity and diversity of active aerobic methanotrophs in the water columns and sediments (0–25 cm) from an arctic tundra lake (Lake Qalluuraq) on the north slope of Alaska and a subarctic taiga lake (Lake Killarney) in Alaska''s interior. The water column CH₄ oxidation potential for these shallow (∼2 m deep) lakes was greatest in hypoxic bottom water from the subarctic lake. The type II methanotroph, Methylocystis, was prevalent in enrichment cultures of planktonic methanotrophs from the water columns. In the sediments, type I methanotrophs (Methylobacter, Methylosoma and Methylomonas) at the sediment-water interface (0–1 cm) were most active in assimilating CH₄, whereas the type I methanotroph Methylobacter and/or type II methanotroph Methylocystis contributed substantially to carbon acquisition in the deeper (15–20 cm) sediments. In addition to methanotrophs, an unexpectedly high abundance of methylotrophs also actively utilized CH₄-derived carbon. This study provides new insight into the identity and activity of methanotrophs in the sediments and water from high-latitude lakes.     

Methane‐derived carbon flow through microbial communities in arctic lake sediments

Aerobic methane (CH₄) oxidation mitigates CH₄ release and is a significant pathway for carbon and energy flow into aquatic food webs. Arctic lakes are responsible for an increasing proportion of global CH₄ emissions, but CH₄ assimilation into the aquatic food web in arctic lakes is poorly understood. Using stable isotope probing (SIP) based on phospholipid fatty acids (PLFA‐SIP) and DNA (DNA‐SIP), we tracked carbon flow quantitatively from CH₄ into sediment microorganisms from an arctic lake with an active CH₄ seepage. When 0.025 mmol CH₄ g^?1 wet sediment was oxidized, approximately 15.8–32.8% of the CH₄‐derived carbon had been incorporated into microorganisms. This CH₄‐derived carbon equated to up to 5.7% of total primary production estimates for Alaskan arctic lakes. Type I methanotrophs, including Methylomonas, Methylobacter and unclassified Methylococcaceae, were most active at CH₄ oxidation in this arctic lake. With increasing distance from the active CH₄ seepage, a greater diversity of bacteria incorporated CH₄‐derived carbon. Actinomycetes were the most quantitatively important microorganisms involved in secondary feeding on CH₄‐derived carbon. These results showed that CH₄ flows through methanotrophs into the broader microbial community and that type I methanotrophs, methylotrophs and actinomycetes are important organisms involved in using CH₄‐derived carbon in arctic freshwater ecosystems.     

Carbon flux and diversity of nematodes and termites in Cameroon forest soils

Theoretically, there are three principal ways in which ecosystem processes might respond to reductions in species richness. These theories are reviewed, and then considered in the context of a study of the diversity of soil nematodes and termites in near-primary forest sites at Mbalmayo, Cameroon, and the contribution made by these two taxa to carbon fluxes (CO₂ and CH₄) from the forest floor. Nematode abundances average 2.04 × 10⁶ m^-2, and termites between 2933 and 6957 m^-2. The site is the most species-rich yet investigated for both groups anywhere in the world, so that a very large number of species contribute to carbon fluxes. We speculate about how much redundancy might be built into the functioning of both assemblages, and point out the enormous difficulties of resolving such questions, and of producing such detailed species-inventories.     

Sources of organic carbon and depositional environment in the Bengal delta plain sediments during the Holocene period

This study investigated the sources of organic matter and sediment depositional environment within fluviatile sediments of the Ganges–Meghna (GM) delta plains. The very low contents of trace metals e.g., chromium (Cr), cobalt (Co), scandium (Sc), and vanadium (V), organic carbon content, and cerium (Ce)-anomaly data of sediments indicate the redox conditions that fall within the boundary of oxic–anoxic condition, with dominantly oxic conditions in the sediment deposition environment. The higher atomic carbon nitrogen (C/N)a ratios and depleted stable carbon isotope ratio (δ¹³C) values for sediments from three study areas indicated the terrestrial sources of organic matter derived from C₃ plant materials, whereas the contribution of organic materials from C₄ vegetation and riverine productivity is low. Some silty sand samples exhibited lower (C/N)a ratios and enriched δ¹³C values in Sonargaon and Faridpur areas that are attributed to the adsorption of ammonium ions on clay minerals and the contribution of organic matter from C₄ plants. Total sulfur over total organic carbon (TS/TOC) ratios in sediments of the Ganges delta reflect the nonmarine environments of sediment deposition. The lower ratios of syringyl to vanillyl phenols (S/V), cinnamyl to vanillyl phenols (C/V), and acid to aldehyde in vanillyl phenols (Ad/Al)v observed in Daudkandi indicate that the lignin in sediments derived from dominant woody gymnosperm sources and is very highly degraded. By contrast, the S/V ratio, C/V ratios, and [Ad/Al]v ratios in Faridpur suggest that the lignin in sediments derived from a mixture of woody and nonwoody angiosperm plant tissue contribution that underwent high degradation as well.     

Methane and soil CO2 production from current‐season photosynthates in a rice paddy exposed to elevated CO2 concentration and soil temperature

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above‐ground assimilation and below‐ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free‐air CO₂ enrichment (FACE) fumigating depleted ¹³CO₂ in rice paddy resulted in a smaller ¹³C/¹²C ratio in plant‐assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO₂ and CH₄) into current‐season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil‐warming treatment nested within the CO₂ treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO₂ treatment, the FACE treatment decreased the ¹³C/¹²C ratio not only in the rice‐plant carbon but also in the soil CO₂ and CH₄. The estimated new C contribution to dissolved CO₂ was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle‐formation stage onwards. For CH₄, the contribution of new C was greater than for heterotrophic CO₂ production; ca. 40–60% of season‐total CH₄ production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH₄ production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH₄ emission through greater rhizodeposition.     

Reconstruction of the carbon balance for microsites in a boreal oligotrophic pine fen, Finland

 Carbon dioxide (CO₂) exchange was studied at flark (minerotrophic hollow), lawn and hummock microsites in an oligotrophic boreal pine fen. Statistical response functions were constructed for the microsites in order to reconstruct the annual CO₂ exchange balance from climate data. Carbon accumulation was estimated from the annual net CO₂ exchange, methane (CH₄) emissions and leaching of carbon. Due to high water tables in the year 1993, the average carbon accumulation at the flark, Eriophorum lawn, Carex lawn and hummock microsites was high, 2.91, 6.08, 2.83 and 2.66 mol C m^–2, respectively, and for the whole peatland it was 5.66 mol m^–2 year^–1. During the maximum primary production period in midsummer, hummocks with low water tables emitted less methane than predicted from the average net ecosystem exchange (NEE), while the Carex lawns emitted slightly more. CH₄ release during that period corresponded to 16% of the contemporary NEE. Annual C accumulation rate did not correlate with annual CH₄ release in the microsites studied, but the total community CO₂ release seemed to be related to CH₄ emissions in the wet microsites, again excluding the hummocks. The dependence of CO₂ exchange dynamics on weather events suggests that daily balances in C accumulation are labile and can change from net carbon uptake to net release, primarily in high hummocks on fens under warmer, drier climatic conditions. Received: 16 August 1996 / Accepted: 30 November 1996